Golden Valley, Minn. — Republican Ron Paul said front-runner Mitt Romney's inability to brush back challenges in votes Tuesday "opens up the door" to him and others looking to deny the former Massachusetts governor the party's presidential nomination.

The Texas congressman was headed for a solid second-place in Minnesota's caucuses, which were won by former Pennsylvania Sen. Rick Santorum. Santorum also won Missouri and was in the hunt for a victory in Colorado's caucuses.

"I'm a little surprised. I thought Romney would have some of this automatic carryover," Paul told The Associated Press ahead of remarks to supporters gathered at a party near Minneapolis. "It certainly doesn't hurt us."

In Colorado, Paul finished fourth with 12 percent of the votes, just behind Gingrich with 13 percent. Romney got 35 percent and Santorum won with 40 percent.

Romney won Minnesota convincingly in his 2008 bid for the GOP nomination. He didn't put much personal effort into the state this time, but he visited once and loaded up on campaign mailings to GOP activists.

In Paul's case, he invested plenty of time and money in Minnesota as he searched for the first win of his campaign. He said he's happy with the results and will shift his sights to Maine, where caucus results are due Saturday.

None of Minnesota's 40 eventual convention delegates were bound by preference ballot results. Paul's advisers said their supporters focused on flooding local party slates; it could give them power to influence the selection of Minnesota's national delegates in May.

"The straw vote is one thing and there's another thing called delegates," Paul told a roaring crowd at his Golden Valley party. "That is where we excelled."

In a brief appearance at Coon Rapids Middle School, Paul waded through a crowd to sign autographs, pose for photos and remind people to vote. He declined to make any predictions on the Minnesota outcome before heading to another suburban Minneapolis caucus site.

Paul said his candidacy was perfectly built for Minnesota. "They love liberty, they don't like wars and they certainly don't like the Federal Reserve," he said.

One caucus-goer in Coon Rapids, truck driver Rod Garberson, said if Paul isn't the fall nominee the Republican Party won't get his vote.

"I gotta tell ya, this time I'm pretty well stuck on not supporting the party even if it means eight years of Obama and we all stand in bread lines for five hours for a sack of potatoes," Garberson said.